1. Field of Invention
The present invention relates to a tension spring, and more particularly, but not exclusively, for use as an artificial prosthetic ligament of capable of large elastic deformation and having high resistance to breakage.
2. Description of Prior Art
The ligaments are biologic structures present in all human articulations, where they are used as stabilizers. As such, they must have a high strength coupled with a capacity for large elastic deformation to sustain repetitively the loads imposed on the articulation.
If the loads cause stresses exceeding the strength of the ligaments, it produces a partial or complete rupture of the ligament. Since most ligaments are not vascularized, any partial or complete rupture cannot heal by itself as a muscle or a bone would do. In most cases of complete rupture of the ligament, the clinician must replace the ligament by either biologic materials, (skin, facia, tendon) or by an artificial prosthesis. Known ligament prosthesis heretofore known have not been very successful.
Artificial ligaments were proposed using one biocompatible material. However, the mechanical behavior of the ligament cannot be mimicked by a single material because of the high strength and large elastic deformation required. For a single material the deformation and strength are related through the modulus of elasticity.
A prosthesis made of a single material would give a result that if the strength of the prosthesis is sufficient, its elastic deformation is not. Therefore, the prosthesis suffers a large plastic deformation. If on the other hand, the material is elastic enough, the strength requirement is not met.
Another problem encountered with artificial prosthesis ligaments is that the attachment points of such ligaments in the bones results in a deterioration of the bones at such attachment points.
In U.S. Pat. No. 3,176,316 issued Apr. 6, 1965 to B. R. Bodell, there is disclosed the construction of a plastic prosthetic tendon. However, this construction is not suitable for use as a ligament as it is an inelastic flexible elongated structure and its purpose is to transmit a force with negligible deformation. On the other hand, a ligament is an elastic flexible elongated structure and its purpose is to stabilize an anatomical joint, and as such, it acts as a spring. Therefore, it must deform under load.
Several types of prostheses are presently implanted in the human knee to replace a ruptured cruciate ligament. None of them, however, have reportedly been adequate due to inacceptable plastic deformations after a relatively short period of time. The functional part of these prostheses is usually made of a single synthetic material with adequate strength properties but not allowing sufficient elastic elongation as compared to the natural ligament.